Soil hydraulic properties and plant water relations: Controls over plant community structure and function in arid environments

Background/Question/Methods Net primary productivity and the distribution of plant species in arid and semi arid regions is tightly linked to water availability as plants are forced to cope with extreme variability in drought inter-drought cycles. Soil texture plays a critical role on how these cycles are manifested across arid landscapes because of its control on soil hydraulic characteristics. Coarse textured soils have larger pore spaces and subsequently higher saturated conductivity than finer textured soils. The higher saturated conductivity typically results in greater infiltration of precipitation and reduced losses to soil evaporation (particularly after summer rain pulses). However, coarse-textured soils lose more water and have lower conductivity at higher soil water potentials than fine-textured soils to the extent that plants growing in coarse soils exhaust their water supply at higher water potentials than plants growing in fine textured soils. Plants adjust to soil hydraulic characteristics through a suite of mechanisms that are reviewed in the context of plant distribution patterns and productivity in the face of climate change. Results/Conclusions Plants occurring on coarse-textured soil may overcome the steep loss of soil hydraulic conductivity by developing higher root to leaf area ratios or by lowering the rate of transpiration. Both features reduce the rate of water uptake per root surface area. Xylem cavitation resistance also correlates with the range of water potentials experienced by the plant, given that there are excess costs that may be associated with highly resistant xylem. Plants on coarse textured soils would therefore be expected to have xylem that is less resistant to cavitation than plants on finer textured soils. A review of the recent literature suggests that these traits generally occur within woody plants. However, xylem cavitation is not always associated with soil texture. In some cases, xylem elements appear “overly” resistant for their environment. One explanation is that high cavitation resistance in shallow roots may facilitate rapid uptake of summer rain that only occurs in low-frequency intense pulses. Alternatively, resistant xylem may facilitate hydraulic redistribution by deep root systems and vice versa. Regardless of the extent of physiological adjustment, soil hydraulic properties will underpin future patterns of plant community structure and ecosystem water balance in the face of global change.